This invention relates to a substrate for a magnetic information storage medium used for an information storage device and, more particularly, to a substrate for magnetic information storage medium such as a magnetic disk made of a glass-ceramic having an improved surface characterisitic including a super flatness suited for a contact recording system and being capable of preventing sticking of a magnetic head to a magnetic information storage medium in a CSS (contact start and stop) type magnetic information storage medium, and a magnetic information storage medium formed by subjecting the magnetic information storage medium substrate to a thin film-forming process.
There is an increasing tendency for utilizing a personal computer for multiple media purpose and a digital video camera with resulting handling of moving image and voice information and this tendency necessitates an information storage medium of a larger recording capacity. For this purpose, the bit number and track density of a magnetic information storage medium must be increased and the size of a bit cell must be reduced for increasing a longitudinal recording density. As for a magnetic head, it must be operated in closer proximity to the surface of the magnetic information storage medium in conformity with the reduction of the bit cell size. In a case where a magnetic head is operated at an extremely low flying height or in a semi-contact state against a magentic information storage medium, there occur sticking of a magnetic head to the surface of a magnetic information storage medium and accompanying head crash and damage to a thin magnetic film at the time of start and stop of the magnetic head.
For overcoming such problem, there is an increasing necessity for development of new techniques for start and stop of a magnetic head including the ramp loading system according to which a magnetic head is completely brought outside of the surface of a magnetic information storage medium at the time of start and stop of the magnetic head and the landing zone system according to which a processing for preventing sticking of a magnetic head is made in a particular portion (i.e., a portion about the inner periphery where no recording or writing of data is made) of a magnetic information storage medium substrate and the magnetic head starts and stops in this particular portion. In the current CSS type information storage device, the magnetic head repeats an operation according to which the magnetic head is in contact with the magnetic disk before starting of operation of the device and is lifted from the magnetic disk when the operation of the device is started. If the surface of contact of the magnetic head with the magnetic disk is a mirror surface, sticking of the magnetic head to the magentic disk occurs with the result that rotation of the magnetic disk is not started smoothly due to increase in friction and damage to the disk surface occurs. Thus, the magnetic disk is required to satisfy two conflicting demands of realizing lowering of the magnetic head necessitated by the increase in the recording capacity on one hand and prevention of sticking of the magnetic head on the other. For satisfying these two conflicting demands, development is under way for the techniques of the ramp loading system according to which the magnetic head is completely brought outside of the magnetic information storage medium and the landing zone system according to which an area for start and stop of the magnetic head is provided in a particular zone of the magnetic informatoin storage medium. Further, not only the current fixed type information storage devices, but also a removable information storage device is being developed for use in, e.g., a digital video camera. From the standpoint of such new technique, characteristics required for a substrate of a magnetic information storage medium are as follows:
(1) In the ramp loading system for a magnetic information storage device, the magnetic head is operated, as a result of increase in the recording capacity, in a semicontact state or in a complete contact state with respect to the surface of a magnetic information storage medium and, for this reason, the magnetic informaton storage medium substrate must have a super flat surface with a surface roughness of 1 .ANG. to 5 .ANG. and the substrate must be capable of being polished to such super flat surface.
(2) In the landing zone system for a magnetic information storage device, the landing zone (i.e., a zone in which the magnetic head starts and stops) must have a surface state which can sufficiently prevent sticking of the magnetic head. For this purpose, projections and depressions must be formed on the surface of the magnetic information storage medium by means of a laser diode pumped solid-state laser or a CO.sub.2 laser.
(3) In the CSS characteristics in the landing zone type magnetic information storage medium, sticking tends to occur between the head and the magnetic information storage medium due to increase in contact resistance caused by a high speed rotation of the magnetic head information storage medium, if the magnetic information storage medium has a smooth surface with surface roughness (Ra) below 50 .ANG. in a landing zone of the medium. On the other hand, if the magnetic information storage medium has a rough surface with surface roughness (Ra) above 300 .ANG. in the landing zone, there is a danger of occurrence of crash of the magnetic head. It is necessary, therefore, to control the height of projections or depressions on the surface in the landing zone to 50 .ANG. to 300 .ANG. and also to control an interval of such projections or depressions in the landing zone to 10 .mu.m to 200 .mu.m and the surface roughness (Ra) in the landing zone to 10 .ANG. to 50 .ANG..
(4) Since the amount of lifting of the head tends to decrease to the order of 0.025 .mu.m or below owing to improvement in the recording density of the magnetic information storage medium, a data zone on the surface of the magnetic information storage medium should have a surface roughness (Ra) of 1 .ANG. to 5 .ANG. which enables the head to maintain this amount of lifting.
(5) By reason of an increased longitudinal recording density, the magnetic information storage medium substrate should not have crystal anisotropy, a foreign matter or other defects and should have a dense, fine and homogeneous texture.
(6) The material of the magnetic information storage medium should have sufficient mechanical strength and hardness for standing a high speed rotation, contact with the head and use as a portable device such as a removable information storage device.
(7) As the longitudinal recording density of a magnetic information storage medium is increased, an extremely fine and precise magnetic thin film such as a perpendicular magnetic recording thin film is required. For satisfying such requirement, the material of the magnetic information storage medium substrate must not contain Na.sub.2 O, K.sub.2 O, B.sub.2 O.sub.3, F or OH group ingredient in principle because these ingredients cause dispersion of ion of such ingredient during the thin film forming process with resulting deterioration in the characteristics of the magnetic thin film.
(8) The mateial of the magnetic information storage medium must have chemical durability against rinsing and etching with various chemicals.
(9) With respect to forming of the magnetic thin film, the magnetic information storage medium substrate should have a low thermal expansion characteristic within the range from -10.times.10.sup.-7 /.degree. C. to 20.times.10.sup.-7 /.degree. C. for avoiding deterioration in the thin film characteristics due to sputtering and heat treatment during the film forming process.
Aluminum alloy has been conventionally used as a material of a magnetic information storage medium substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness. Further, since an aluminum alloy is a soft material, deformation tends to take place so that it cannot cope with the recent requirement for making the magnetic information storage medium such as a magnetic disk thinner and the requirement for high density recording because the magnetic information storage medium tends to be deformed by contact with the head with resulting damage to the recording contents.
As a material for overcoming this problem of the aluminum alloy substrate, known in the art are glass substate for magnetic information storage medium made of a chemically tempered glass such as a sodium lime glass (SiO.sub.2 --CaO--Na.sub.2 O) and alumino-silicate glass (SiO.sub.2 --Al.sub.2 O.sub.3 --Na.sub.2 O). These glass substrates, however, have the following disadvantages:
(1) Polishing is made after chemical tempering and so the tempered layer tends to cause instability in thinning the magnetic information storage medium.
(2) For improving the CSS characteristics, the substrate must be subjected to texturing which produce projections and depressions on the surface of the substrate. Since a mechanical processing or a thermal processing such as by laser beam cause a cracking or other defects due to distortion in the chemically tempered layer, texturing must be conducted by a chemical etching or sputtering but this prevents a large scale production of the product at a competitive cost.
(3) Since the Na.sub.2 O or K.sub.2 O ingredient is included as an essential ingredient in the glass, the magnetic thin film characteristics of the glass is deteriorated with the result that the substrate cannot cope with the requirement for increasing the surface recording density.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some substrates made of glass-ceramics. For example, Japanese Patent Application Laid-open No. 6-329440 discloses a glass-ceramic of a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 system which includes lithium disilicate (Li.sub.2 O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2) as predominant crystal phases. This glass-ceramic is an excellent material in that, by controlling the grain size of globular grains of .alpha.-quartz, the conventional mechanical texturing or chemical texturing becomes unnecessary and surface roughness (Ra) of a polished surface can be controlled within a range from 15 .ANG. to 50 .ANG.. This glass-ceramic cannot cope with the above described target surface roughness (Ra) of 1 .ANG. to 5 .ANG. and also cannot sufficiently cope with the above described tendency to lowering the flying height of a magnetic head necessitated by rapid increase in the recording capacity. Besides, in this glass-ceramic, no discussion about the landing zone to be described later in this specification is made at all.
Japanese Patent Application Laid-open No. 7-169048 discloses a photo-sensitive glass-ceramic of a SiO.sub.2 --Li.sub.2 O system including Au and Ag as photo-sensitive metals characterized in that a data zone and a landing zone are formed on the surface of a magnetic disk substrate. A predominant crystal phase of this glass-ceramic is lithium silicate (Li.sub.2 O.SiO.sub.2) and/or lithium disilicate (Li.sub.2 O 2SiO.sub.2). In case lithium silicate is used, the glass-ceramic has a poor chemical durability so that it has a serious practical problem. Further, in forming of the landing zone, a part of the substrate (i.e., landing zone) is crystallized and is subjected to chemical etching by using 6% solution of HF. However, forming of the substrate with an uncrystallized part and a crystallized part makes the substrate instable mechanically as well as chemically. As for chemical etching by HF solution, it is difficult to control concentration of the HF solution because of evaporation and other reasons so that this method is not suitable for a large scale production of products.
Several methods are known for forming a landing zone and a data zone on the surface of a magnetic information storage medium. For example, Japanese Patent Applicaton Laid-open No. 6-290452 discloses a method for forming a landing zone on a carbon substrate by a pulsed laser having a wavelength of 523 nm. In this case, however, there are the following problems:
(1) A carbon substrate is formed by pressing at a high pressure and burning at a high temperature of about 2600.degree. C. with resulting difficulty in a large scale production at a low cost.
(2) A carbon substrate is low in its mechanical properties (Young's modulus and fracture strength) so that it is difficult to cope with the thinning tendency and a high speed rotation of a drive.
(3) The forming of the landing zone utilizes oxidization and evaporation of carbon by the pulsed laser. Since carbon is a material which causes a very strong thermal oxidation reaction, the formed landing zone becomes instable and thus it poses a serious problem in reproduceability.
Japanese Patent Application Laid-open No. 7-65359 and U.S. Pat. No. 5062021 disclose a method for forming a landing zone on an aluminum alloy by a pulsed laser. The aluminum alloy has the above described problems. Besides, the surface of the substrate after irradiation of laser beam tends to have a defect due to oxidation of a molten portion and remaining of splash of molten metal on the surface. It is therefore difficult to put this method to a practical use.
Japanese Patent Application Laid-open No. 63-46622 discloses a hard disk for magnetic information storage using a pluality of low thermal expansion materials. This reference presents only a discussion about thermal distortion. As usable materials, 96% silica glass and quartz glass are shown but these materials are low in mechanical strength and are not very practical. Further, glass-ceramics which are presented as usable are generally known ones having the Na.sub.2 O ingredient. No discussion is made in this reference about the problem about a magnetic thin film which is indispensable to cope with the tendency to high recording density. Neither is there discussion about the surface roughness of the surface of the substrate or the techniques about the ramp loading and the landing zone.
Japanese Patent Application Laid-open No. 6-92681 and Japanese Patent Application Laid-open No. 8-133783 disclose SiO.sub.2 --Al.sub.2 O.sub.3 --SiO.sub.2 system low-expansion transparent glass-ceramics. These references are not aware of utility of such glass-ceramics as a magnetic information storage medium substrate and no discussion is made about the super flat surface of the substrate.
It is therefore an object of the invention to eliminate the disadvantages of the prior art technique and provide a magnetic information storage medium substrate which is suitable for the two techniques coping with the tendency to high recording density and has a super flat surface characteristic on the entire surface of the substrate which is particularly for the ramp loading system and also has two surface characteristics which enable a stable lifting of a magnetic head in a landing zone where the magnetic head starts and stops (contact start and stop) and enable a low flying height of the magnetic head in a data zone in conformity to the tendency to high recording density.
It is another object of the invention to provide a magnetic information storage medium formed by subjecting the magnetic information storage medium substrate to a thin film forming process.